Optimize sourcemap application more (#25425)

* Optimize sourcemap application more

* Remove test-only memory hog sourceMapDecodedMappings field

* Update for style, remove unused function that triggers warnings in node 10

* Avoid all raw buffer constructor calls

* Small TDZ fix

src/testRunner/parallel/host.ts

@@ -20,6 +20,7 @@ namespace Harness.Parallel.Host {
         // tslint:disable-next-line:variable-name
         const FailedTestReporter = require(path.resolve(__dirname, "../../scripts/failed-tests")) as typeof import("../../../scripts/failed-tests");
 
+        const perfdataFileNameFragment = ".parallelperf";
         const perfData = readSavedPerfData(configOption);
         const newTasks: Task[] = [];
         let tasks: Task[] = [];
@@ -175,8 +176,6 @@ namespace Harness.Parallel.Host {
             }
         }
 
-        const perfdataFileNameFragment = ".parallelperf";
-
         function perfdataFileName(target?: string) {
             return `${perfdataFileNameFragment}${target ? `.${target}` : ""}.json`;
         }

src/testRunner/unittests/convertToBase64.ts

@@ -2,7 +2,7 @@ namespace ts {
     describe("convertToBase64", () => {
         function runTest(input: string): void {
             const actual = convertToBase64(input);
-            const expected = new Buffer(input).toString("base64");
+            const expected = (Buffer.from ? Buffer.from(input) : new Buffer(input)).toString("base64");
             assert.equal(actual, expected, "Encoded string using convertToBase64 does not match buffer.toString('base64')");
         }